Signed in as soon as I read this. Pulisa is right, however much you care for your MIL, this isn't your mum. Concentrate on helping Mr.C through the shock he is feeling right now. I know myself the sheer terror you can feel when you think your elderly parents are dying and you know this too :( His world is crumbling but being strong will come easier to you than you think. We can be an anxious wreck yet from somewhere deep inside an unknown strength will surface to carry you through and you will carry Mr C. through with you.
Give him my very best wishes at this awful time and take care, both of you